I saw a clip of video titled “David Letterman came back to The Late Show this week,” on Time Magazine's website. In the CBC’s show, Letterman revealed that he was threatened by a jihadist lately. After joking he wishes the audience to be more like a human shield for him on the site, he said:

A guy, a radical extremist threatened me to cut my tongue up. I wished I had a nickel for every time a guy has threatened to cut.

I know a nickel means a small change, but small change wouldn’t save you from a great threat. What does "I wished I had a nickel for every time he was threatened to cut" exactly mean — particularly in contrast to the seriousness (or gravity) of likening the audience to the human shield of himself against jihadists even as a humor?

Oishi-san: The others have explained the "I wish I had a nickel ..." trope well, but it is worth noting additionally that the joke David Letterman is making there is based on irony, a reversal of the literal meaning of the words. The fact is, very few people ever get threatened in such a manner. So when he makes such a bald statement, it's a surprise because it's really not true. And so it gets a laugh. Remember, David Letterman is all about getting laughs.
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RobustoAug 27 '11 at 1:23

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Additionally, I think he is making a pun. "To cut" can mean to physically cut someone with a knife, but it can also mean to cancel a show.
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Kit Z. Fox♦Aug 27 '11 at 3:34

2 Answers
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The phrase "I wish I had a nickel for every time X happened" is used to mean that it happens so often that you would be rich. Other phrases are "If I had a dollar for every time X happened, I'd be rich!" It can be used with any amount of money, but using a smaller amount can indicate that the event happens so often that even a small amount builds up.

It has nothing to do with extremists themselves, and it can be used for many things. In this case, it means that being threatened by radical extremists happens often enough that the speaker could make money off it.

The phrase is also common enough that you could adapt it for conversation. For example, if you're always losing your glasses you might tell a friend:

If I had a dollar for every time I lost my glasses, I'd be able to buy a pair for every day of the year!

Letterman was making a joke, but it does seem in poor taste compared with creating a human shield.

It means that, if he had (accumulated) a nickel for every time, he would now have a sizable amount of money. It's generally used as way to exaggerate, such as "if I had a penny for every time X, I'd be able to retire by now". Or, for comic effect: "if I had a nickel for every woman who fell for me, I'd be able to buy a piece of bubble gum" or "If I had a dime for every time a homeless guy asked me for change, I'd still say no."